MgB2 under pressure: phonon calculations, Raman spectroscopy, and optical reflectance

The effect of pressure on optical phonon frequencies of MgB2 has been calculated using the frozen-phonon approach based on a pseudopotential method. Grüneisen parameters of the harmonic mode frequencies are reported for the high-frequency zone-centre E2g and B1g and the zone-boundary E2u and B2u modes at A. Anharmonic effects of phonon frequencies and the implications of the calculated phonon frequency shifts for the pressure dependence of the superconducting transition temperature of MgB2 are discussed. Also reported are Raman and optical reflectance spectra of MgB2 measured at high pressures. The experimental observations in combination with calculated results indicate that the broad spectral features that we observed in the Raman spectra at frequencies between 500 and 900 cm-1 cannot be attributed to first-order scattering by zone-centre modes, but originate in part from a chemical species other than MgB2 at the sample surface and in part from a maximum in the MgB2 phonon density of states. Low-temperature Raman spectra taken at ambient pressure showed increased scattering intensity in the region below 300 cm-1.